Anekoik Room

Anechoic chamber is a specially designed acoustic test room in which echoes or reflections are eliminated. The term “anechoic,” of Greek origin, means “non-reflective.” These chambers are primarily used to simulate free-field conditions for the propagation of sound and electromagnetic waves. Anechoic chambers serve as essential tools in various industrial tests, research and development processes, and measurements requiring compliance with international standards.

^{Image of an Anechoic Chamber – TÜBİTAK UME, GEBZE (}TÜBİTAK UME⁾

Anechoic chambers are equipped with high-performance sound and wave absorption materials mounted on interior surfaces. These absorbers are typically wedge-shaped and installed on all six surfaces (walls, ceiling, and floor), effectively eliminating nearly all reflections from the chamber’s surfaces. The floor is often supported by a transparent platform made of tensioned wire mesh; however, semi-anechoic solutions with reflective hard floors are also preferred for practical reasons and to reduce measurement errors.

In the design of sound absorbers, material density and resistance to airflow are critical parameters. To achieve effective absorption at low frequencies, wedge structures may be made multi-layered or incorporate resonator-based designs. In electromagnetic applications, materials such as carbon-loaded foam or ferrite tiles are commonly used.

Anechoic chambers are used for measurements in both acoustic and electromagnetic fields. In acoustic chambers:

• The sound power levels of machinery and equipment can be determined.
• The directional and spectral characteristics of noise sources can be analyzed.
• Sound components emitted by product subsystems can be individually examined.

RF anechoic chambers used for EMC (Electromagnetic Compatibility) testing are designed to:

• Perform emission and susceptibility tests,
• Provide indoor conditions that can replace open-area test sites (OATS),
• Enable standardized measurements at 3-meter and 10-meter test distances.

Measurements conducted in anechoic chambers are defined by international standards such as ISO 3745 and ANSI S12.35. These standards specify methods for measuring the room’s anechoic performance, microphone placement, sound source characteristics, and measurement procedures. The directivity of the sound sources used during chamber validation is critical to the reliability of measurement results.

^{Image of an Anechoic Chamber – İ.T.Ü. OTAM ()}

Fully anechoic environments are not always necessary and can be costly. Therefore, semi-anechoic chambers with reflective floors are widely used. Such chambers provide a more practical and suitable test environment, especially for measuring devices that interact with the ground, such as washing machines, compressors, and electric tools.

The history of anechoic chambers dates back to the mid-20th century, when radio waves and acoustic properties began to be studied scientifically. With the widespread adoption of EMC regulations in the 1980s, major corporations shifted toward constructing chambers as alternatives to open-area test sites. A key milestone was the installation in 1982 of the first 3-meter EMC anechoic chamber for IBM. From the 1990s onward, the use of ferrite tiles and hybrid absorber materials enabled smaller chamber sizes with improved performance.

Measurements in anechoic chambers vary depending on the method used and the size of the test object. The two primary measurement approaches defined in international acoustic standards are:

• Half-Sphere Method: In this method, the test object is assumed to be at the center of a half-sphere, with microphones placed around it at specific distances and angles to capture the directional characteristics of the radiated sound. This method is particularly suitable for small and symmetric sources.
• Rectangular Prism Method: Preferred for large machinery, this method uses microphones arranged around a rectangular prism enclosing the test object. It is well-suited for non-directional or complex source structures.

In both methods, sound power levels are calculated using A-weighted sound pressure levels, octave or 1/3-octave band analyses, and frequency components.

The performance of anechoic chambers depends on the properties of the absorber materials installed on interior surfaces. The primary purpose of these materials is to absorb sound or electromagnetic waves without reflection. In acoustic anechoic chambers, absorbers typically take the following forms:

• Wedge-Type Absorbers: The traditional design provides high absorption at low and mid frequencies. Longer wedges improve performance at lower frequencies, but their large volume and space requirements can complicate installation.
• Layered and Resonator-Based Absorbers: Resonant structures can be integrated into the base of absorbers to enhance low-frequency performance, providing additional absorption at lower frequencies.
• Hybrid Absorbers: In electromagnetic chambers, combinations of ferrite tiles and carbon-loaded foam offer more compact solutions. Ferrite tiles are effective in the 30–100 MHz range, while foam-based absorbers dominate at higher frequencies.

Material selection is determined by the target frequency range, chamber size, and specific testing requirements.

Anechoic chamber design is determined by parameters such as the target frequency range, required measurement accuracy, and the size of the objects to be tested. Key design considerations include:

• Frequency Range: The length of absorber material and the internal chamber volume are determined based on the lowest frequency of interest. Lower frequencies require longer absorbers and larger internal volumes.
• Precision Class: As defined by ISO standards, measurement classes are “Precision (Grade 1),” “Engineering (Grade 2),” or “Survey (Grade 3).” Precision-class chambers provide more homogeneous free-field conditions and operate with tighter tolerances.
• Structural Isolation: The chamber is constructed using a “room-within-a-room” principle to prevent internal sound from escaping and external noise from interfering. Sound transmission between inner and outer structures must be blocked, and solid contact avoided.
• Vibration and Noise Isolation: The chamber is mounted on floating floor systems (neoprene pads or spring-based systems) to isolate it from structural vibrations, preventing measurement contamination from building-borne noise.
• Geometric Shape and Microphone Placement: The chamber’s shape may be designed with non-parallel surfaces to minimize internal reflections. Microphone arrangements follow standardized half-sphere or rectangular prism configurations.

All these parameters ensure high-accuracy measurements in both acoustic and electromagnetic testing.

Anechoic chambers are used across various industries for product development, quality control, and regulatory compliance testing. Each sector may require different types of anechoic chamber solutions based on specific measurement needs. Major application areas include:

• Automotive Industry: Sound power levels and radiation patterns of subsystems such as engines, fans, and air conditioning units are measured. Electromagnetic immunity tests for in-vehicle systems are also conducted in RF anechoic chambers. Such tests are part of type approval processes for products targeting international markets.
• White Goods and Home Electronics: Noise emissions from products such as washing machines, vacuum cleaners, and small appliances are tested under specific operating modes in anechoic environments, enabling certification of their sound levels in compliance with standards.
• Medical Devices: Electromagnetic immunity tests for sensitive equipment such as magnetic resonance (MR) imaging systems are performed in anechoic chambers. Compliance with safety criteria is evaluated for both electromagnetic emissions and acoustic performance.
• Defense and Aerospace: Directional radiation tests for radar and antenna systems are conducted in anechoic environments. These measurements enhance system accuracy by allowing wave behavior to be studied without reflections.
• Information Technology and Telecommunications: Electromagnetic emissions from computers and network devices are measured in EMC anechoic chambers to ensure compliance with regulations such as FCC and CE. Antenna characterization is also performed in these environments.

These application examples demonstrate that anechoic chambers are not merely academic tools but integral components of product development and certification processes.

^{Image of a Vehicle Test in an Anechoic Chamber ()}

In Türkiye, anechoic chamber infrastructure has developed to meet growing testing demands in industry and academic research centers. These chambers enable domestic products to meet the EMC and acoustic compliance requirements necessary for international market acceptance. Key institutional examples include:

• TÜBİTAK UME (National Metrology Institute): Located in Gebze, the center houses both reverberation and anechoic chambers. TÜBİTAK UME is a leading institution in Türkiye for certifying anechoic and reverberation chambers according to ISO standards and providing measurement validation services.
• İstanbul Technical University – OTAM (Automotive Technologies Research Center): OTAM, located at the Maslak campus, possesses anechoic test chambers for acoustic and vibration testing in the automotive industry. Sound power levels of engines, exhaust systems, and climate control units are measured in detail here.
• Hema Endüstri (Çerkezköy): Anechoic chambers used for industrial product development and testing are employed primarily in acoustic performance validation processes tailored to customer requirements.

In addition to these institutions, major domestic manufacturers such as Vestel, BSH, and Arçelik also maintain anechoic and semi-anechoic chambers to control the acoustic characteristics of their products. This infrastructure has become an indispensable part of product development, quality control, and compliance evaluation processes.